Ford Focus Electrical and Lighting Problems?

My 04 Ford Focus Zetec had a strange electric problem. I am away for 4 weeks, when I started the engine there is a variation in the lights and heater fan. I mean, all the lights including the headlights and parking lights brightens up and goes down with out acceleration and the heater fans also goes up and down. I drove for 4 days and I never had any problems with the engine. What could be the reason?